Security facsimile paper serves to prevent unauthorized persons from reading information printed on a facsimile transmission. U.S. Pat. No. 5,001,749 to Iggulden et al. teaches a commonly used facsimile security paper. The paper includes a lower layer of plain white "bond" or similar paper. This is the medium on which the images from the facsimile machine are most clearly printed, and is indeed the medium to be read by the intended recipient. A cover medium of thin polyester or other material is disposed over the plain paper before and during the printing by the facsimile machine. The cover medium should be removed only by the intended recipient of the facsimile transmission. The cover medium includes a bottom surface facing and contacting the plain paper. This bottom surface is coated with a wax-based or resin-based ink which melts onto specific locations on the plain paper in response to heating from the print head of the facsimile machine. This process forms letters and other symbols or images on the paper. The cover medium includes a top surface facing away from the plain paper. Thus, the cover functions like a ribbon of the type used in old typewriters. Unfortunately the cover medium, like a ribbon, can be read after imprinting--though with some difficulty. Iggulden attempts to overcome this problem by overprinting the top surface with an interference pattern of inked letters or the like for purposes of obscuring what has been printed on the plain paper through the cover medium.
Overprinting an inked interference pattern on the top surface of the cover medium has two main disadvantages: first, if the ink is thick enough to produce an obscuring effect, the ink will interfere with the thermal printing process and a ghost image of the overprinted interference pattern will appear on the images printed on the plain paper; moreover, overprinting with an ink-based interference pattern is not very effective in obscuring what has been printed on the cover medium and the plain paper. This is because the process of melting ink from the cover medium to the paper at localized spots creates a differential reflectivity. At the same time, the thermal print head deforms the cover medium somewhat in the same localized spots. This two-fold "marking" of the cover medium renders it readable in certain lighting conditions. In such conditions, an unauthorized person can still read--though with some difficulty--what has been printed on the sandwich of plain paper and cover medium, regardless of any ink overprinting.